Fish weight calculator: length to pounds

How does this calculator turn length into weight?

Fish do not grow as cubes. If they did, doubling the length would multiply weight by exactly eight, and every species would share one formula. What fisheries science publishes instead is a standard-weight equation fitted separately to each species:

log10(Ws) = a + b × log10(TL), with weight in grams and total length in millimetres.

The exponent b is the interesting part. For largemouth bass it is 3.273, so a long bass gains weight faster than the cube law predicts. For walleye it is 3.18, and for channel catfish 3.294. Those third-decimal differences are why a 24-inch bass and a 24-inch walleye are not close in weight, and why one generic calculator cannot serve both.

The output of the equation is Ws, the standard weight — specifically the 75th percentile weight for that species at that length. A fish sitting exactly on the curve is in good condition, not average condition. The body-condition control scales that figure by relative weight, so choosing "Lean" applies a factor of 0.85 and "Heavy" applies 1.15.

Where do the equations come from?

Each species uses a different published source, which is worth knowing because they were not all built the same way. The channel catfish equation comes from Brown, Murphy and Springer's 1995 paper in theJournal of Freshwater Ecology, fitted across 241 populations. The walleye equation traces to Murphy, Brown and Springer in 1990 and was built from 42,487 individual fish across 114 populations. The largemouth bass equation is Henson's from 1991.

All three are regression-line-percentile fits, the standard method in North American fisheries work. We take the coefficients from the FSA standard-weight database maintained at Northland College rather than retyping them from a secondary blog post, and re-check them on the schedule set out in ourfact-checking policy.

What lengths matter on the Upper Mississippi?

Keepers and photograph fish are not the same lengths across species here. A fifteen-inch walleye is a legal fish in both pools; a thirty-inch channel cat is the story fish in the Lock and Dam 8 tailwater; a twenty-inch largemouth is where the five-pound argument starts in the backwaters. The table below runs the same lengths for all three so you can see how fast the curves diverge. For species notes the hub cannot carry, use thecatfish weight calculator, thewalleye weight calculator, or theour bass weight calculator.

What does a Mississippi River fish of this length weigh?

Here is the same calculation run at six common lengths, with every fish set to typical condition. Read it as the middle of a range, not a target.

Look at how the gap opens. At 12 inches the three species sit within six ounces of one another: 0.5 lb for the catfish, 0.6 lb for the walleye, 0.9 lb for the bass. At 32 inches the catfish is 13.5 lb, the walleye 13.9 lb, and the bass 21.9 lb. Same starting point, wildly different finish, entirely because of a difference in the third decimal place of the exponent. Anglers who learned a rule of thumb on one species and carried it to another are usually wrong in exactly this direction.

Two worked examples

A 26-inch walleye from the Lock and Dam 8 tailwater

Convert first: 26 in × 25.4 = 660.4 mm. Thenlog10(Ws) = −5.453 + 3.18 × log10(660.4). The log of 660.4 is 2.8199, so the right-hand side is −5.453 + 8.967 = 3.514, and Ws = 10^3.514 = 3,266 g, or 7.20 lb. A fish that thick in early spring is usually pre-spawn and heavier than the curve, so setting condition to Heavy returns 8.28 lb, with a range of 7.56 to 9.00 lb.

An 18-inch channel catfish from a Pool 9 backwater

18 in is 457.2 mm. log10(Ws) = −5.8 + 3.294 × log10(457.2) gives −5.8 + 8.7624 = 2.9624, so Ws = 917 g, or 2.02 lb. Late-summer channel cats feeding on gizzard shad in the backwaters often run heavy; at 115 relative weight the estimate rises to 2.32 lb. The lean setting on the same fish gives 1.72 lb, so the condition control moves the answer by almost ten ounces — worth remembering before you argue with someone about a scale.

What do anglers get wrong about length-to-weight tables?

The most common mistake is treating a single number as a measurement. A printed chart that says "24 inch walleye = 5 lb" is quoting one point on a distribution that, on this very equation, runs from 4.74 lb for a lean fish to 6.42 lb for a heavy one. Nothing about the chart is wrong; the certainty people read into it is.

Second is the fork-length substitution. Channel catfish and walleye both have forked tails, and measuring to the fork rather than the squeezed tip costs you two to four percent of the length — which, run through an exponent above three, costs you closer to ten percent of the weight.

Third is the girth formula. Weight = girth² × length ÷ 800 circulates widely, and nobody who posts it can name the study it came from. It is a rough carryover from saltwater practice. We left it out rather than dress an unsourced rule up as science. If you want the condition of your fish reflected in the answer, the relative-weight setting does that job with a citation attached.

Fourth, and specific to this river: fish weigh differently through the season. A walleye full of eggs in late March and the same walleye in June are two different weights at one length. Ourwalleye weight calculator covers that seasonal swing in more detail than a shared page can.